High performance microprocessor, microcontroller and communication chips require very high speed interconnects between the active transistor devices which are used to perform the various functions such as logical operations, storing and retrieving data, providing control signals, and the like. With progress in the transistor device technology leading to the present ultra large scale integration, the overall speed of operation of these advanced chips is beginning to be limited by the signal propagation delay in the interconnection wires between the individual devices on the chips. The signal propagation delay in the interconnects is dependent on the RC product wherein, R denotes the resistance of the interconnect wires and C represents the overall capacitance of the interconnect scheme in which the wires are embedded. Use of copper instead of aluminum as the interconnect wiring material has allowed the reduction of the resistance contribution to the RC product. The current focus in the microelectronics industry is to reduce interconnect capacitance by the use of lower dielectric constant (k) insulators in building the multilayered interconnect structures on chips.
One prior art method of creating interconnect wiring network on such small a scale is the dual damascene (DD) process schematically shown in FIGS. 1a to 1g. Referring to FIG. 1a, In the standard DD process, an inter metal dielectric (IMD), shown as two layers 1110, 1120 is coated on the substrate 1100. The via level dielectric 1110 and the line level dielectric 1120 are shown separately for clarity of the process flow description. In general, these two layers can be made of the same or different insulating films and in the former case applied as a single monolithic layer. A hard mask layer or a layered stack 1130 is optionally employed to facilitate etch selectivity and to serve as a polish stop. The wiring interconnect network consists of two types of features: line features that traverse a distance across the chip, and the via features which connect lines in different levels of interconnects in a multilevel stack together. Historically, both layers are made from an inorganic glass such as silicon dioxide (SiO2) or a fluorinated silica glass (FSG) film deposited by plasma enhanced chemical vapor deposition (PECVD).
Referring to FIG. 1b and FIG. 1c, in the dual damascene process, the position of the lines 1150 and the vias 1170 are defined lithographically in photoresist layers 1500 and 1510 respectively, and transferred into the hard mask and IMD layers using reactive ion etching processes. The process sequence shown in FIGS. 1a through 1d is called a “line-first” approach. After the trench formation, lithography is used to define a via pattern 1170 in the photoresist layer 1510 and the pattern is transferred into the dielectric material to generate a via opening 1180, as illustrated in FIG. 1d. 
The dual damascene trench and via structure 1190 is shown in FIG. 1e after the photoresist has been stripped.
As shown in FIG. 1f, the recessed structure 1190 is then coated with a conducting liner material or material stack 1200 that serves to protect the conductor metal lines and vias and serve as an adhesion layer between the conductor and the IMD. This recess is then filled with a conducting fill material 1210 over the surface of the patterned substrate. The fill is most commonly accomplished by electroplating of copper although other methods such as chemical vapor deposition (CVD) and other materials such as aluminum or gold can also be used. The fill and liner materials are then chemical-mechanical polished (CMP) to be coplanar with the surface of the hard mask and the structure at this stage is shown in FIG. 1f. A capping material 1220 is deposited as a blanket film, as is depicted in FIG. 1g to passivate the exposed metal surface and to serve as a diffusion barrier between the metal and any additional IMD layers to be deposited over them. Silicon nitride, silicon carbide, and silicon carbonitride films deposited by PECVD are typically used as the capping material 1220. This process sequence is repeated for each level of the interconnects on the device. Since two interconnect features are simultaneously defined to form a conductor inlaid within an insulator by a single polish step, this process is designated a dual damascene process.
In order to lower the capacitance, it is necessary to use lower k dielectrics such as PECVD or spin-on organo-silicates which have k values in the 2.5 to 3.1 range instead of the PECVD silicon dioxide based dielectrics (k=3.6 to 4.1). These organosilicates have a silica like backbone with hydrogen and/or organic groups such as alkyl or aryl groups attached directly to the Si atoms in the network. Their elemental compositions generally consist of Si, C, O, and H in various ratios. The C and H are most often present in the form of methyl groups (—CH3). The primary function of these methyl groups is to add hydrophobicity to the materials. A secondary function is to create free volume in these films and reduce their polarizability. The k value can be further reduced to 2.2 (ultra low k) and even below 2.0 (extremely low k) by introduction of porosity in these insulators. For the purpose of brevity, these ultra low k and extreme low k materials will be referred to collectively as very low k materials in this document.
Although a tunable range of k values is possible with this set of very low k materials, there are several difficulties in integrating these materials with copper interconnects by the dual damascene process described above or by any other variation of the dual damascene process. The chief difficulty is that the organosilicate-based materials are very sensitive to plasma exposures because of the relative ease of oxidation or cleavage of the Si-organic group linkage (for example, Si-methyl) which results in formation of silanol (Si—OH) groups in the film through a potential reaction with moisture in the ambient environment. Silanols absorb water and hence increase the dielectric constant and the dielectric loss factor of the film significantly, thus negating the performance benefits expected from the very low k films. They also increase the electrical leakage in the film and thus create a potentially unreliable interconnect structure. Since reactive ion etch and plasma etch are key steps required in the formation of the dual damascene trench and via structure as described above, and in the removal of photoresists used in patterning the very low k materials, it is very difficult, if not impossible, to avoid plasma damage of this class of films during a prior art dual damascene integration.
Several attempts have been made to minimize the loss of hydrophobicity in the low k films using non-oxidizing resist strip plasmas consisting of some or all of He, H2, N2, CO etc. However, it must be noted that none of these plasma chemistries completely succeed in preventing the loss of hydrophobicity of the very low k materials. This is especially the case for porous low k materials which have a very large surface area and are easily susceptible to damage during the resist strip processes.
Another method to prevent the low k material from losing its hydrophobicity and its dielectric properties is the use of fluorinated or non-fluorinated organic polymer based low k materials such as Dow Chemical's SiLKT™ dielectric, Honeywell's Flare™ and other polyimides, benzocyclobutene, polybenzoxazoles, aromatic thermoset polymers based on polyphenylene ethers; and chemical vapor deposited polymers such as poly paraxylylene which are not susceptible to damage during traditional process plasma exposures associated with the dual damascene processing. However, these materials do not possess the other properties required of a low k dielectric film such as a low thermal expansion and small pore sizes.
Another problem facing the successful integration of organosilicate-based porous materials is that they are very fragile mechanically due to their low elastic modulus, fracture toughness and hardness which often lead to failures in CMP, dicing and packaging operations. The mechanical strength of these resins depends on both the void volume as well as their chemical structure. Their mechanical strength decreases with increasing porosity as well as increasing cage-like structure of the siloxane backbone. Since it is imperative that a low dielectric constant be maintained, it is very difficult to decrease the void volume while maintaining the same mechanical strength.
Several methods (Padhi et al., J. Electrochem. Soc., 150 (1), G10–G14, (2003), and U.S. patent application publication U.S. 2004/0087135 A1 of Canaperi et al, assigned to the same assignee as that of the present invention) have been proposed to handle porous organosilicate materials with weak mechanical strengths but most of these methods are difficult to implement due to the fact that these methods either involve a nonstandard process flow or a nonstandard tool. Therefore they are expensive to implement in production.
In the literature on porous silica based films, (For example, Prakash et al., Nature, 374, 439, (1995)), surface modification to introduce hydrophobic end groups during film formation is accomplished by means of a wet chemical treatment wherein the silylating agent (Tri-methyl chloro silane—TMCS) is introduced into the porous network by means of a low surface tension carrier solvent. Such a reaction, called silylation, is feasible for films that are in the process of forming since there is a great deal of free volume and an abundance of silanols that would otherwise condense and bridge. Thus far, it is not clear whether a similar reaction can be performed on fully formed films which, even after exposure to process chemistries that damage the film, have fewer silanols than the films that are in the process of forming. There have been studies published by Chang et. al., (J. Electrochem Soc., 149, 8, F81–F84, 2002) where an attempt has been made to recover the hydrophobicity and the carbon content of the porous OSG film after damage using hexamethy disilazane (HMDS) as the silylating agent. However, it is clear from their results that HMDS in any medium is unable to recover, completely, the properties of the porous OSG film. Similarly, TMCS is not completely effective at recovering the dielectric properties either. Both EMDS and TMCS are monofunctional silylating agents with the ability to attack only a single isolated silanol group per molecule on the surface and pore wall of the low k material. However, organosilicate based low k materials have two distinct types of silanols which are classified as follows (Gun'ko et. al., J. Colloid and Interface Sci 228, 157–170 (2000)): The first type of silanol is the non-hydrogen bonded silanol which in itself consists of, (1) completely non-interacting single silanols (also called isolated silanols) which do not have any neighboring silanols nearby, (2) very weakly interacting silanols, and (3) weakly and non-interacting geminal silanols (also called disilanol). The second type of silanol is the hydrogen bonded silanol. Most monofunctional silylation agents attack and replace the isolated silanols readily, but generally do not attack the other two types of non-hydrogen bonded silanols as readily. The primary reason for this is that steric hindrance prevents the simultaneous capture of more than one silanol with a monofunctional silylating agent readily. Additionally, it is also important to use a silylating agent with the most reactive functionality to readily silylate the surface and pore walls of the low k material without releasing a byproduct of the reaction that is corrosive.
Hu et al., (J. of Electrochem. Soc., 150 (4) F61–F66 (2003)) have also published a study where they examine the efficacy of dimethyldichloro silane (DMDCS) as a silylating agent to recover the properties of low k materials. However, in their study, they report that dimethyldichloro silane forms a monolayer on the top surface of the film and does not penetrate the bulk of the porous low k material. Thus, unless the appropriate silylating medium as well as conditions for the silylation are used, it is difficult to recover the bulk dielectric properties of the low k material. Additionally, the byproduct of any chlorine based silylating agent such as dimethyldichloro silane and TMCS is hydrogen chloride, which is corrosive and cannot be used in interconnect structures that contain copper.